Iron intermetallic compounds

Suitable metallic fillers may be composed of aluminum, copper, magnesium, chromium, tin, nickel, silver, iron, titanium, and alloys, such as stainless steels, bronzes. In addition, metal fillers may comprise intermetallic chemical compounds, such as titanium diboride and carbides of the above metals. Further, conductive non-metal fillers may comprise tin oxide, indium tin oxide, etc (7). 

Figure A1.37 shows the iron-carbon phase diagram up to 6.7 wt% carbon (to the first intermetallic compound, FejC). Of all the phase diagrams you, as an engineer, will encounter, this is the most important. So much so that you simply have to learn the names of the phases, and the approximate regimes of composition and temperature they occupy. The phases are ...

It has been seen that iron has an adverse effect because it forms a second phase (insoluble) material in the alloy which acts as an effective local cathode. Sequestering is the technique of adding an alloying addition that will cause an alternative intermetallic compound with iron to form. This compound might form a dross to be removed mechanically. Alternatively the new intermetallic compound could be a less effective cathode in which case removal would not be necessary. 

Both silicon and aluminium are added to zinc to control the adverse effects of iron. The former forms a ferro-silicon dross (which may be removed during casting). Aluminium forms an intermetallic compound which is less active as a cathode than FeZn,] . Similarly in aluminium and magnesium alloys, manganese is added to control the iron . Thus in aluminium alloys for example, the cathodic activity of, FeAl, is avoided by transformation of FeAlj to (Fe, Mn)Al/. This material is believed to have a corrosion potential close to that of the matrix and is, therefore, unable to produce significant cathodic activity . 

Typically, Be-containing alloys and intermetallic phases have been prepared in beryllia or alumina crucibles Mg-containing products have been synthesized in graphite, magnesia or alumina crucibles. Alloys and compounds containing Ca, Sr and Ba have been synthesized in alumina , boron nitride, zircon, molybdenum, iron , or steel crucibles. Both zircon and molybdenum are satisfactory only for alloys with low group-IIA metal content and are replaced by boron nitride and iron, respectively, for group-IIA metal-rich systems . Crucibles are sealed in silica, quartz, iron or steel vessels, usually under either vacuum or purified inert cover gas in a few cases, the samples were melted under a halide flux . 

It is very difficult to cool pure metals and other pure elements fast enough to form glasses. However, metallic alloys can often be converted into glasses, particularly if they contain a mixture of small and large atoms such as iron and boron, or they are multi-component mixtures of metals that crystallize into more than one intermetallic compound (i.e., eutectic compositions). Thus, covalent chemical interactions of the atoms are important because they stabilize liquids and thereby inhibit crystallization. 

Indium combines with several metals, such as sodium, potassium, magnesium, iron, palladium, platinum, lanthanium and cerium, forming semiconductor-type intermetallic compounds. 

Titanium iron hydrides are among the materials which, at the present time, appear to have potential for practical applications as an energy-storage medium (7). The formation and properties of titanium iron hydride have been studied by Reilly and Wiswall (3), who found that the reaction proceeds in two steps as indicated by Reactions 5 and 6. Both hydrides have dissociation pressures above 1 atm at room temperature in contrast to TiH2 which is very stable. Titanium iron is representative of intermetallic compounds that consist of an element (titanium) capable of forming a stable hydride and another element (iron) that is not a hydride former or at best, forms a hydride with great difficulty. Iron presumably plays a role in destabilizing the hydrides. Titanium also forms a 1 1 compound with copper (there are other intermetallic compounds in the titanium-copper system) and this fact, coupled with the observation that copper... 

Intermetallic and Semiconducting Compounds. Indium forms intermetallic compounds with a great many metals and combinations of metals including alkali metals, magnesium, the iron group, rare earths, and precious metals such as the platinum group. Carbon-free indium-based... 

The most familiar metals are elemental substances such as iron, tin, aluminium etc. However, many compounds are metallic. As well as intermetallic compounds such as AgCd and NaTl, and a huge number of non-stoichiometric alloys, many oxides, sulphides, halides etc. have metallic properties. For details of structure and bonding in metallic substances, see Section 7.5. 

Figure 11.4.6 shows the pressure (P) versus composition (x) isotherms for the hydrogen-iron-titanium system. This system is an example of the formation of a ternary hydride from an intermetallic compound. 

Fig. 1.2. Optical micrographs of the transition zone between commercial purity iron or steel and aluminium. The Fe2Al5 intermetallic compound layer becomes progressively flatter and thinner with increasing carbon content of steel. The A1 + an intermetallic compound eutectic is distributed at grain boundaries of aluminium solid solutions.

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